Laser ablation of cryogenic films: Implications to Matrix-Assisted Pulsed Laser Deposition of biopolymers and dedicated applications in nanotechnology

Matrix-Assisted-Pulsed Laser- Evaporation (MAPLE) is evolving into a powerful technique for the controlled deposition of biopolymers. The method relies on the finding that the biopolymers dissolved within a frozen, absorbing solvent can, upon laser irradiation, eject in the gas phase in intact and functional form. For the better mechanistic understanding and optimization of the technique, we review here studies on laser-induced material ejection from cryogenic solids of simple molecular/ organic compounds. It is demonstrated that at low laser fluences, thermal desorption dominates, so that, only weakly-bound-to-the-solvent dopants/ solutes desorb. However, above a specific fluence, a different mechanism becomes operative, resulting in the unselective ejection of a layer of material (ablation). Ejection of solutes that are strongly bound to the solvent e. g. of biopolymers, can take place only in this regime. Therefore, the term evaporation in MAPLE acronym (and "desorption" in MALDI) are inappropriate. At least for photoinert compounds and nanosecond laser pulses, ablation is shown to be due to explosive boiling. We discuss the implications of this mechanism for MAPLE. Chemical effects in UV ablation of frozen compounds are reviewed and conditions for their minimization are presented. Finally, besides their mechanistic interest, the studies can also yield information on a number of issues relevant to nanotechnology.